This invention relates generally to imaging lens systems, and more particularly the invention relates to the characterization of aberrations in an optical lens and lens system and to visual testing and integrated circuit mask analysis.
In the fabrication of electronic integrated circuits, line patterns of less than a tenth of a micron are now being developed. This is less than 1/500 the width of a human hair. At these dimensions, the projection printing of mask patterns in integrated circuit layout can be adversely impacted by small residual aberrations in the lens system. While the quality metric (Strehl ratio) of today's projection printers is within a few percent of unity, residual aberrations still contribute significant spillover of signals from one mask opening in another. These spillover effects degrade the image quality with position within the field of the die. Good correlation of measured aberrations with the difference in horizontal and vertical linewidth along the slit in a scanning system has been observed. Such aberration-based linewidth variations are themselves partially mitigated by higher image slopes created through optical proximity correction (OPC). Yet residual cross-chip linewidth variations suggest that residual aberrations continue to contribute a level of degradation that is about half as large as the level of improvement gained through applying OPC. The impact of these aberration-based spillover effects will clearly become more important with phase shifting masks due to the inherent use of more coherent illumination as well as the presence of both phases to more efficiently direct energy to a broader set of locations in the lens pupil. Since lithography at low k1 and high NA is adversely affected by small residual aberrations in lenses even though Strehl ratios exceed 0.98, test targets are needed to act as ‘canaries’ that are more sensitive than product features as well as to quantify individual aberrations to 0.01 λrms.